Extrusion blow molding permits the fabrication of bodies of a rather intricate geometric design to be formed from a simple, extruded tube. The wall thickness of the hollow body thus produced depends on that of the tube and on the blow ratio. It it true that wall thickness of the hollow body may be additionally influenced by nozzle configuration and the regulation of the wall thickness of the extruded tube; the range of variation that is obtainable in that way is, however, too small for certain applications and the gradation of wall thickness necessary for fabricating ribs or the like cannot be attained.
The patent DT-OS 1,479,563 as well as the patent DT-AS 1,176,829 teaches the use of blow molds with movable parts that can be actuated for the molding of handles in canisters or for turning the inside of the bottom of hollow bodies, respectively. The patent Fr-PS 1,441,727, the patent DT-AS 1,130 151, as well as the patent DT-AS 1,245,579, disclose movable parts of blow molds by means of which reinforcing ribs may be formed.
According to the patent FR-PS 1,441,727, the movable tool parts are withdrawn before the blowing starts, so that pocket-like enlargements are provided that the tube wall enters into during the blowing. By advancing these parts before cooling the plastic material, the intruded wall area is folded and welded at the regions of contact.
According to the patent DT-AS 1,130,151 as well as the patent DT-AS 1,245,579, tool parts that limit an enlargement of the mold pocket are so advanced against one another, after the blowing and before the cooling of the thermoplastic material, that the material blown into the enlargement is upset into a rib or into an aperture, respectively.
In order to keep open and to calibrate the aperture of a hollow body, it is know, for example, from patent DT-AS 1, 176,829 to insert a mandrel into this aperture. In order to cut apertures in hollow bodies after their cooling a multiple cutting ring has been proposed by the patent DT-OS 2,163,144 in conjunction with a bearing ring which may also be located in the blow itself. Especially in cutting in the mold, the periphery of the aperture in the hollow body has to be so designed that it can spring back in cutting direction. Cutting tools of the described design have, therefore, only a limited applicability.
The patent DT-AS 1,100,936 as well as the patent DT-AS 1,174,968 describe blow molds with contrivances for the severing of a wall area of a hollow body in order to form a defined aperture. A disadvantage that appears is that the cutting parts of the tool are guided by supporting surfaces, so that the specific lift has to be held within close tolerances and is, therefore, critical.
In connection with mass production items, especially on drinking cups, one tries to work with only a limited amount of material. In the case of extrusion blowing, to be sure, it is possible to obtain minimal wall thickness of the product if the customary thermoplastic plastics are processed in such a manner that the plastic tube is extruded with a small wall thickness; because of the desired mechanical properties of the hollow body, there exist, however, limits to the reduction of the wall thickness that cannot be exceeded through contouring, e.g. upset ribs.
Furthermore, in a number of cases it is found to be undesirable that hollow bodies of slight wall thickness are poor thermal insulators: For example, thin walled cups filled with hot beverages can only be briefly held in the hand without unpleasant heat effects; and ice cream filled into such cups melts in them with undesirable speed because of the absorption of heat. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide apparatus for the extrusion blowing of hollow bodies that permits the fabrication of mechanically stable and thermally insulating hollow bodies using a minimum amount of material.
With this and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.